Introduction

A tutorial intended to help developers port their Qt3/KDE3 based applications to Qt4/KDE4. Porting a KDE3 based application needn't be a difficult process. Already, there are many scripts and sources of documentation which can help.

Conventions

For the instructions in this tutorial we use the following shorthand:

program refers to an executable program

path refers to a path

file refers to a file

$SVN is the fullpath to the location of your KDE subversion checkout

CMake

Unlike KDE3, KDE4 applications will be built with the help of CMake. The easiest way to port your autotools system to CMake is with the help of the am2cmake script which can be found in the cmake/scripts directory of the kdesdk module. This will create a series of CMakeLists.txt files alongside your old buildsystem files.

For example, if your source code is located in /path/to/src then:
% cd /path/to/src
% $SVN/trunk/KDE/kdesdk/cmake/scripts/am2cmake --kde4
Run am2cmake --help to check whether you need the --kde4 flag.

There is also a tool that looks in your resulting CMakeList.txt files for potential problems. This tool called cmakelint.pl is located in $SVN/trunk/kde/kdesdk/scripts. Use it like so:
% cd /path/to/src
% $SVN/trunk/KDE/kdesdk/scripts/cmakelint.pl CMakeLists.txt
Or, to run it over your entire source directory:
% cd /path/to/src
% find . -name CMakeLists.txt | \

xargs $SVN/trunk/KDE/kdesdk/scripts/cmakelint.pl

Qt4 API

An overview of the Qt3 to Qt4 transition is provided in Trolltech's
"Moving from Qt 3 to Qt 4" paper. This document provides an excellent overview into the major functionality changes with Qt4 and is highly recommended.

The follow-on "Porting to Qt 4" page gives an amazingly detailed description of the porting process, along with a list of the changes in the classes and functions.

These documents describe a tool provided by Trolltech called qt3to4 that can help port the Qt parts of your code from Qt3 to Qt4, using compatibility functions. Run qt3to4 as follows:
% $SVN/trunk/qt-copy/bin/qt3to4 [options] <Infile>, [Infile], ...
Infile can be a source file or a project file. If you specify a project file, ending with '.pro' or '.pri', qt3to4 will port all files specified in that project.

Additionally, there is a program called remove-qt3-support.pl in the kdesdk module that will search and replace lots of deprecated Qt3 stuff for you. Simply run this program without any options in the source directory.
% $SVN/trunk/KDE/kdesdk/scripts/qt4/remove-qt3-support.pl

KDE4 API

Much of the porting effort consists of simply renaming class names and header files. Since it would be rather tedious to change all these by hand, there is a handy script in the scripts/qt4 directory of kdesdk called adapt-to-kde4-api.pl. This will scan all your files and create a diff output which can then be used to patch your code.

Once that simple code substitution has been done, you will still have to go through your code to port to, for example, the new KAction API. Documentation about all API changes is kept in the KDE4PORTING.html file in the kdelibs module.

Qt Designer UI Files

Qt designer ".ui" files created using Qt3 must be converted to the new Qt4 format. This can be done using the uic3 program available from your Qt4 installation.

Icons

KDE4 uses the freedesktop.org icon naming specification as the basis for icon names. This means that both the icons that ship with KDE4 (Oxygen) as well as components in kdelibs that use icons follow this specification.

Porting your app from the icon names used in KDE3 to the ones used in KDE4 is as easy as running the adapt-to-icon-spec.py script from the root directory of your project and follow the instructions on screen.

The script automatically converts confirmable positives (e.g. uses of KIcon or KIconLoader), skips confirmable negatives and prompts for what to do with possible positives. It shows the latter with additonal context if desired and makes it a simple matter of pressing 'y' or 'n' for the possible hits to complete the porting.

Internationalization

To create your ".pot" file, copy the commands from the 'messages' rule in your projects Makefile.am to a shell script called Messages.sh. You may assume the same variables ($PREPARETIPS, $XGETTEXT, $podir, etc.) still exist, but keep in mind the differences between Makefile and shell script syntax.

Also be careful that if you use the -k parameter with $XGETTEXT, you will need to explicitely list all variants that you use.

Do NOT play with frames of groupboxes, labels, or lineedits to fake a different widget. Use the appropriate widget instead. e.g., instead of setting a label to have a sunken lineedit border, how about using a readonly KLineEdit instead? And instead of using a KLineEdit without a border for a copyable widget, use KActiveLabel.

Do NOT use a groupbox without border to group widgets! Just use a layout.